Large metal working presses are used in many stamping operations and usually consist of a ram which is brought into contact with a metal part placed in a die mounted on an upper or lower platen for forming the metal into the desired shape. In certain presses the die is mounted on the moving ram. These presses impart thousands of pounds of pressure during the forming stroke to form the metal member into the desired configuration. Most of this pressure or force is absorbed by the metal. However there is always a certain amount of excess force or energy that is transferred and absorbed by the supporting die, the press platen and other components of the press. Unless the die platen, etc. are supported on resilient mounts or springs, this excess energy applied to the press quickly results in cracks and breakage of the press and various components thereof. Therefore to reduce this fatigue problem the die and/or platen as well as the hydraulic or pneumatic ram is supported on some type of shock absorbing device such as springs.
One common type of spring used in many processes is a nitrogen die spring in which a piston is slideably mounted in a cylinder containing high pressure nitrogen. The press forces or excess energy is applied to the piston rod which compresses the nitrogen within the cylinder to absorb the forces preventing their transfer directly to the press components. These nitrogen die springs possess the serious problem of requiring a sliding seal between the piston and cylinder walls. Such seals are difficult to maintain in a gas tight condition due to the high pressures of the nitrogen and the constant sliding movement of the seal along the wall of the cylinder.
Nitrogen is the desired gas for such springs since it provides a more linear compression rate than most gasses, including air, and it enables the desired results applicable at various pressures, to be easily calculated and achieved, and is not subject to breakdown or wide changes in temperatures as are other gasses. However, this requires the aditional expense of supplying and maintaining the nitrogen for the springs.
This sliding seal problem has been attempted to be eliminated by the use of self-contained die cushions which use an elastomeric bladder filled with nitrogen or other compressible gas. Although these die cushions eliminate the sliding seal problem, they require a considerable large size bladder to achieve the desired pressure which requires larger components to be built into the press or other piece of equipment for receiving the die cushion. Thus even though the sliding seal problem has been eliminated by these die cushions, they possess other undesirable characteristics, namely size vs. pressure ratio.
The other known prior art related to the present invention is disclosed in the following patents.
U.S. Pat. No. 894,117 is the closest known prior art and discloses a pneumatic suspension device using a flexible sleeve attached by a screw tightened clamping plate within the interior of an inner member with the other end of the sleeve clamped by a screw tightened plate to the inside of an outer member telescopically movably mounted with respect to the inner member. The spring of this patent requires a considerable number of components, such as stop washers, screw actuated sealing clamps, spring biased lubricating pads, expensive machined and/or cast metal parts, and requires the rolled portion of the sleeve to extend about a lower edge of a movable inner sleeve which would result in rapid wear and a short life span to the flexible sleeve.
Swiss Pat. No. 216,813 discloses a telescopic shock absorbing assembly in which a piston has a flexible bellows attached to it by an annular crimping member. The other end of the flexible member is attached at an annular ring at the bottom of the stationary lower housing. A movable upper housing telescopes outwardly about the outer housing and a flexible member rolls between the interior surface of the housing and the exterior surface of the piston.
Russian Pat. No. 423,927 discloses a flexible bellows which has an outer member which telescopes upwardly around an inner stationary member when the fluid pressure between the bellows and inner member is pressurized and expands the bellows upwardly.
One end of the bellows member is attached to a portion of a telescoping member by an annular crimping ring.
German Pat. No. 2,751,430 discloses another type of device in which a flexible bellows is attached at one end between a relatively stationary upper and lower housing and at its upper end to an annular piston plate.
None of these prior art devices are able to achieve the results of the present invention set forth below by a spring construction similar to that disclosed herein. Therefore the need exists for an improved die spring construction which is able to withstand high repeated loads exerted thereon without the use of internal sliding seals and which will enable air to be used as the compressible fluid if desired, and which can be accomplished with a considerable fewer components than required in known prior art devices.